1. Field of the Invention
This invention relates to a method for molding a connecting plastic sleeve integral with one end of a plastic corrugated drainpipe for use in civil construction work or agriculture, e.g.
2. Background of the Invention
A plastic corrugated pipe 1 shown in FIG. 5 is increasingly used as a drainpipe in civil construction work or agriculture. The corrugated pipe 1 is of a double-wall structure and has an outer wall 11 formed of alternate annular ridges 111 and annular grooves 112. An inner wall 12 is fusion-bonded to the grooves 112. Such a corrugated pipe 1 is not only more pressure resistant but is also flexible while relatively light in weight and allows water to smoothly flow therethrough. Due to the above advantages, the corrugated pipe 1 is readily usable as a drainpipe. A variation in which a water passagehole is made in the portion where the inner wall 12 is fusion-bonded to the groove 112 is particularly recommended as a drainpipe for underdrainage.
Two corrugated pipes are usually connected together using a tube joint. It may sometimes be required, however, to connect two pipes in such a manner as to insert one pipe into a sleeve, which is a receptacle integral with an end of the other pipe 1.
When the connecting sleeve is formed at one end of a conventional cylindrical plastic pipe, the sleeve end is heated to over the softening temperature and sectional dies for forming the sleeve are forced into that end to enlarge the diameter. However the above conventional method applied to the formation of a sleeve at the end of the corrugated pipe 1, as shown in FIG. 6 with a sectional die 2, presents the following problems. If the end of the corrugated pipe 1 is heated too fast, the air enclosed in between the ridge 111 of the outer wall 11 and the inner wall 12 will be rapidly expanded by heat and the pressure thus developed may damage the ridge 111 or inner wall 12. Moreover, if the end thereof is made too soft by excessive heating or if the sectional dies are expanded too strongly or too fast, the ridge 111 will be deformed or put out of shape. The thus degraded external appearance will reduce its commercial value by a large margin. Particularly, in the case of a corrugated pipe having water passageholes, the external appearance will be deteriorated further because the water passageholes are forced to enlarge and, when the pipe is buried under the ground, a great deal of earth and sand is allowed to penetrate into the pipe through the water passageholes and the pipe may become clogged with it. When use is made of the conventional method of expanding the diameter at the end of the corrugated pipe 1 to form a sleeve, the end thereof must be treated carefully, that is, it must be heated gently and expanded slowly by the sleeve forming segmental dies. In consequence, it takes at least more than five minutes to form a sleeve. Accordingly, extremely low forming efficiency and difficulties in setting forming conditions have reduced production efficiency.
The present inventors developed a method for molding a sleeve by injection molding as shown in FIG. 7 to solve the above problems and filed a patent application with the Japanese Patent Office (Japanese Published Patent Application No. 175013/86. The above method comprises preheating and applying pressure to one end 1a of a corrugated pipe 1 to squash two ridges to form a circular pipe. The circularized pipe end of the corrugated pipe 1 is inserted into a mold 3 equipped with a cavity 31 for molding a sleeve. Molten resin 4a is injected from the resin injection hole 32 into the cavity 31 to mold a sleeve 4, as shown in FIG. 8, which is integral with the end 1a of the corrugated pipe 1. Since this method requires no process of expanding the diameter of the end of a corrugated pipe as in the case of the conventional one, it offers high forming efficiency and is fit for mass-production because a sleeve can be formed in less then one minute. Notwithstanding, there is still room for improvement from the following standpoints.
In the first place, the end 1a of the corrugated pipe 1 must be preheated with pressure or cut by a lathe to squash the ridges to form a circular pipe, which results in an increase in the number of production steps.
In the second place, the contact between the end 1a of the corrugated pipe 1 and the outer clamps 33 of the mold 3 is insufficient since the ridges at the end 1a thereof are squashed to form the circular pipe. Accordingly, the molten resin may leak out through the small gap between the end 1a of the corrugated pipe 1 and the outer clamps 33 during the injection of the molten resin. The molten resin thus leaked out forms a flash 41 at the root of the molded sleeve 4, as shown in FIG. 8, thus presenting an ugly appearance. Moreover, the wall thickness of the sleeve 4 is caused to slightly vary because the quantity of the resin being injected into the cavity 31 of the mold 3 for molding the sleeve is not precisely fixed.
Another improvement that should be made is that, since the ridges at the end 1a of the corrugated pipe 1 are squashed to form the circular pipe, the outer periphery thereof tends to be scarcely caught by the mold 3. Therefore, the end 1a of the corrugated pipe 1 may be forced back with the pressure applied during the injection of the molten resin. As a result, the sleeve may be molded unsatisfactorily.
Still another improvement that should be made is concerned with the concentration of heat deformation. When the sleeve 4 is injection-molded in such a manner as to envelop the end 1a of the corrugated pipe 1, heat deformation tends to be concentrated at the root of the sleeve 4. Consequently, if the end 1a of the corrugated pipe 1 is thin, the root of the sleeve 4 is readily broken out on receiving a shock.